Wastewater treatment plant comprising upflow anaerobic reactor, and wastewater treatment method using thereof

ABSTRACT

The present invention provides a method and system for the treatment of wastewater. One embodiment includes an anaerobic reactor and aerobic reactor. A terminal settling tank includes an upflow anaerobic reactor comprising an inlet mounted at the external lowest region of the anaerobic reactor. An introducing mechanism is directly connected to the inlet and located at the lower region in the anaerobic reactor and has a perforated drain pipe therein. A mixer is provided at the main shaft. A discharge is provided for allowing reaction water, which contains some portion of sludge generate by the mixer, to flow into the aerobic reactor. A sludge-accumulator is provided for collecting the sludge, which is settled by gravity during the reaction and produced by the mixer, to the center of the anaerobic reactor.

TECHNICAL FILED

Be present invention relates to a wastewater treatment plant comprisingan upflow anaerobic reactor and to a wastewater treatment method usingthereof. More specifically, the present invention relates to awastewater treatment plant which comprises the facultative bacteria andan anaerobic reactor, and also relates to a method for wastewatertreatment using thereof. By using the present invention, the organicwastewater containing non-biodegradable and toxic matter, nutrients (N,P), and heavy metals can be treated in a stable manner; the sludgetreatment cost can be reduced by keeping a high concentration of sludge;and the land usage for the facilities, the costs for construction andoperation can be much saved by using the preinstalled settling tank as areactor.

BACKGROUND ART

Generally, a wastewater contains nutrients such as nitrogen or phosphor,non-biodegradable and toxic matter, and heavy metals. The treatmentmethods for clarifying such wastewater can be generally classified intothe biological method or treatment of the wastewater containing organiccompounds, and the physicochemical method for treatment of thewastewater having non-biodegradable and toxic matter and heavy metalstherein.

To repeat, for removal of organic matters, the biological method is inthe most common use among the above treatment methods. As arepresentative of the biological treatments, there is an activatedsludge method that employs a treatment plant consisting of an initialsettling tank, an aeration tank, and a terminal-settling tank. In thiscase, however, a thickening tank is indispensably required for thepost-treatment process of the sludge since the sludge concentration inthe settling tank is as low as 5000˜15000 mg/L, and further, theefficiency decline is inevitably invited in the terminal settling tankif under the low concentration of the sludge, for the aeration tank isrun with 1500˜3000 mg/L of the sludge concentration during theoperation. Moreover, in the activated sludge method, there has oftenbeen the sludge-bulking phenomenon by the variation of quality andamount of the influent water, with the result of process failure orwater quality deterioration.

Another biological method for releasing nitrogen or phosphor is oneemploying the anaerobic•aerobic method, in which an aerobic reactorprovided with oxygen thereinto and an anaerobic reactor without thesupply of oxygen are arranged in a row for operation in the conventionalactivated sludge method, and thereby, nitrogen and phosphor can beremoved by means of the action of the microorganisms. However, sincethis method employs a complete-mixing type of reactor, it is difficultto retain high concentration of the microorganisms in the reactor and acomplete anaerobic atmosphere can hardly be kept during denitration orin the guidance of phosphor effluence. Such points as these have oftenserved as restrictions in designing a reactor and brought about theproblem of complicated operation.

In order to treat the wastewater containing non-biodegradable and toxicmatter and heavy metals, it is common to employ a biological methodadded to the physicochemical treatment method. However, suchphysicochemical methods suffer from a drawback in that the operationcost is increased due to the great amount of consumption of thechemicals for adsorption or oxidation. Further, the biological treatmentassociated with these physicochemical methods, in terms of practicaluse, is in its early stage; a secondary treatment is indispensablyrequired for elimination of the intermediate products of heavy toxicitywhich are produced during the initial treatment process; the operationcost is inevitably increased in the anaerobic treatment because of thenecessity of temperature retention.

To sum up, the organic wastewater containing heavy metals has beenclarified by using the physicochemical treatment method in the main, butin recent years, a biological treatment method using thesulfate-reducing bacteria has newly been on the rise. In this method,however, there is a defect that the operation cost is increased due tothe temperature retention, since the sulfate-reducing bacteria, onaccount of its nature, must be grown in a rigorous anaerobic atmosphere.

In conclusion, a more efficient and economic method for treatment ofwastewater has been wanted, in which such problems as inherent to theconventional physicochemical and biological methods for treatment of theorganic wastewater containing non-biodegradable and toxic matter,nutrients such as nitrogen or phosphor, and heavy metals, viz., theproblems in that the cost for treatment is unduly increased, the degreeof clarification has not reached completion as yet, and a secondarytreatment is necessary due to the production of intermediate products,can all be remedied.

The inventors of the present invention have unceasingly endeavored withstudying and doing a lot of research for solving the problems of theprior art, to finally confirm that by using the wastewater treatmentplant which comprises an upflow anaerobic reactor having the improvedintroducing means and discharging means, and combined with theconventional aerobic reactor and settling tank, the organic wastewatercontaining non-biodegradable and toxic matter, nutrients such asnitrogen or phosphor, and heavy metals can successfully be treated,resulting in a greatly improved clarification efficiency. To this end,the inventors have finally accomplished the present invention.

Ultimately, it is the main object of the present invention to provide awastewater treatment plant comprising an upflow anaerobic reactor.

Another object of the present invention is providing of a wastewatertreatment method using said wastewater treatment plant.

SUMMARY OF THE INVENTION

Below, the configuration of the wastewater treatment plant comprising anupflow anaerobic reactor according to the present invention is describedin more detail.

With respect to the wastewater treatment plant consisting of ananaerobic reactor, an aerobic reactor, and a terminal settling tank, thepresent invention is characterized in that said anaerobic reactorcomprises an inlet mounted at the external lowest region of theanaerobic reactor; an introducing means which is directly connected tosaid inlet but is located inside of the anaerobic reactor at the lowerpart, and has a perforated drain pipe therein; the mixing means which isinstalled at the main shaft thereof at a regular interval in theanaerobic reactor; a discharging means which has the radial-shaped weirsextending outwards from the center and is provided at the upper regionin the anaerobic reactor for allowing the reaction water which comes tocontain some portion of the sludge in the reaction generated by saidmixing means, to flow into the aerobic reactor, a sludge-accumulatingmeans which is situated at the lower region in the anaerobic reactor andcollects the sludge at the center in the anaerobic reactor, said sludgebeing settled by the gravity force during said reaction by the mixingmeans.

As one embodiment of the present invention, a wastewater treatment plantemploying the upflow anaerobic reactor according to the presentinvention is described hereinbelow with reference to FIG. 1a.

First of all, as aforementioned, the upflow anaerobic reactor (10)comprises an inlet (11), an introducing means (12), a discharging means(13), a mixing means (14), and a sludge-accumulating means (15).

Of these constituents, the inlet (11) is installed at the externallowest region of the anaerobic reactor (10) in order to introduce boththe influent water and the sludge into the anaerobic reactor (10), saidsludge having been returned from the terminal settling tank (30) by theactuation force of the sludge-returning pump (50). Such inlet (11) isintegrated into a single channel before the introducing means (12) inorder for the influent water to be mixed with the return sludge prior toflowing into the anaerobic reactor.

Next, the introducing means (12), directly connected to said inlet (11),is positioned at the lower region in the anaerobic reactor (10), toallow the influent water or the return sludge passing through said inlet(11) to be introduced into the lower region in the anaerobic reactor(10). The introducing means (12) is shaped as a perforated drain pipe (apipe having perforations thereon) for prevention of the clogging bysludge (ref.: FIGS. 2a˜2 c), and is preferably made of stainless steel.

The discharging means (13) is positioned at the upper region in theanaerobic reactor (10) and is designed so that the reaction water whichcontains some portion of the sufficiently grown sludge by the aid of themixing means (to be described below) can flow into the aerobic reactor(20). Different from the conventional one, the discharging means (13) ofthe present invention has such a configuration in that the weirs areinstalled in a radial manner with being extended outwardly from thecenter, for the purposes of the prevention of clogging by the sludge andthe uniform discharge of the effluent water (ref.: FIGS. 3a˜3 c). Thisdischarging means (13) is designed in such a manner to evenly collectand discharge the reaction water containing the return sludge mixed inthe influent water. And, it is made of an ordinary steel plate.

Next, three to six pieces of the mixing means (14) are installed at themain shaft (17) in the anaerobic reactor (10) at a regular interval ofabout 50 cm˜1 m, to prevent the sludge channeling and mix the reactionwater in a continuous manner, thereby allowing the active fermentationreaction by the microorganisms to be carried out in the anaerobicatmosphere. In this way, the organic wastewater containingnon-biodegradable and toxic matter and nutrients (N, P) is efficiently

purified in the anaerobic reactor (10). The sludge which is settledduring the reaction generated by said mixing means (14) is collected atthe lower center in the anaerobic reactor (10) through thesludge-accumulating means (15), and then discharged outside the reactorby the actuation force of the sludge-discharging pump (40).

Another embodiment of the present invention comprises a wastewatertreatment plant employing the anaerobic reactor in which the introducingmeans (12′) for the influent water and that (12″) for the return sludgeare separately provided. The description thereof is given below withreference to FIG. 1b.

In the second embodiment of the present invention, the inlet (11′) forintroduction of the influent water is separated from the other inlet(11″) for introduction of the return sludge. Correspondingly, the inflowwater-introducing-means (12′) is installed at the lowest region in theupflow anaerobic reactor (10), while the return sludge-introducing-means(12″) which is used for removing the organic matters and nutrition saltscontained in the influent water is installed straight over the inflowwater-introducing-means (12′) in the upflow anaerobic reactor (10).These means are constructed of a perforated drain pipe, which canprevent clogging, by the sludge. Further, the upflow anaerobic reactor(10) of the second embodiment according to the present invention alsocomprises a discharging means (13), a mixing means (14), asludge-accumulating means (15), etc., as illustrated in FIG. 1a.

Another embodiment according to the present invention comprises awastewater treatment plant employing an additional upflow anaerobicreactor 10′), which is installed in a row in the wastewater treatmentplant of the second embodiment of FIG. 1b. A detailed description isgiven below referring to FIG. 1c.

In the third embodiment according to the present invention, anadditional upflow anaerobic reactor (10′) is employed, which isinstalled in a row in the wastewater treatment plant of the secondembodiment of FIG. 1b. In the additional upflow anaerobic reactor (10′),in order to mix the return sludge which is returned from the aerobicreactor (20) via the internal return pump (60), with the reaction waterwhich is discharged from the upflow anaerobic reactor (10) and containssome portion of the sludge, an inlet (11′″) is provided at the externallowest region of the upflow anaerobic reactor (10′). Further, anintroducing means (12′″) is provided at the lower region in theanaerobic reactor (10′) and connected to said inlet (11′″) by means ofthe flanges, thereby allowing both the effluent which is discharged fromthe anaerobic reactor (10) and passed through the inlet (11′″), and thereturn sludge which is returned by the actuation force of the internalreturn pump (60) and passed through the inlet (11′″), to be introducedinto the lower region in the anaerobic reactor (10′). Such introducingmeans (12′″) is constructed of a perforated drain pipe for prevention ofclogging by the sludge (ref.: FIGS. 2a˜2 c). Further, the additionalanaerobic reactor (10′) also comprises a discharging means (13′) and amixing means (14′). Said discharging means (13′) is situated at theupper region in the anaerobic reactor (10′) for discharging the reactionwater containing some portion of the sludge which is sufficientlymatured by the mixing means (14′) (this mixing means (14′) to bedescribed from below) into the aerobic reactor (20). Further, saiddischarging means (13′) has the radial-shaped weirs extending outwardsfrom the center, for the purpose of preventing clogging by the sludgeand allowing the effluent water to be uniformly discharged. As to themixing means (14′), three to six pieces of the nixing means (14′) areinstalled at the main shaft (17′) thereof at a regular interval of about50 cm˜1 m in the anaerobic reactor (10′), to prevent the sludgechanneling and to mix the reaction water continuously, thereby effectingthe active fermentation of the microorganisms in the anaerobicatmosphere. In this way, the organic wastewater containing nutrients (N,P) is efficiently clarified in the anaerobic reactor (10′).

The wastewater treatment plants in all embodiments as described abovecomprise an aerobic reactor (20) and a terminal settling tank (30) inaddition to said anaerobic reactor (10). The aerobic reactor (20) usedherein has an oxygen-generating means, namely, a diffuser (21) at itslower region, for attempting to supply sufficient amount of oxygen forthe facultative fermenting bacteria in order for them to exhibit a highactivity. In this regard, any aerobic reactor (20) of such material andconstruction as commonly used in the art may be suitable for use in thepresent invention. The terminal settling tank (30) is used for removingthe sludge which has been effectively degraded by the facultativebacteria present in the reaction water in said aerobic reactor (20). Forthis, a gravity settling tank is preferred in which the sludge is slowlyremoved by the gravity force in clarifying the wastewater for obtainmentof purified water. In addition, however, such a settling tank asequipped with a scum baffle can also be used for the present invention.The sludge precipitated in the settling tank is collected by thesludge-accumulating means (15), then returned to the inlet (11) at theexternal lowest region of the anaerobic reactor (10) by thesludge-returning pump (50), and finally separated and discharged in thestate of sludge by the sludge-discharging pump (40) in said anaerobicreactor (10). Further, an actuating means (16, 16′) is mounted at theupper end of the main shaft (17, 17′) in the anaerobic reactor (10,10′), to provide the actuation force for running the mixing means (14,14′) connected to the main shaft in the anaerobic reactor.

As described hereinbefore, the wastewater treatment plant according tothe present invention has such an advantageous point in that the organicwastewater containing non-biodegradable and toxic matter and nutrients(N, P) can be treated more efficiently than in the conventionalwastewater treatment plants, by virtue of using the facultative bacteriaand an upflow anaerobic reactor.

Below, the wastewater treatment method which employs the wastewatertreatment plant comprising the upflow anaerobic reactor according to thepresent invention is described in more detail by stages as well as thefunction and effect of said wastewater treatment plant.

Step 1

Introduction of Influent Water into the Anaerobic Reactor

The influent water which is an organic wastewater containingnon-biodegradable and toxic matter, nutrients (N, P) or the like isfirstly guided into the anaerobic reactor (10) through the inlet (11,11′) provided at the external lowest region of the anaerobic reactor(10), and then, said influent water is introduced into the anaerobicreactor (10) through the introducing means (12, 12′) which is connectedto the inlet (11, 11′) inside the anaerobic reactor (10).

Step 2

Fermentation in the Anaerobic Reactor

The activated sludge is introduced into the anaerobic reactor (10)through said inlet (11, 11″) and introducing means (12, 12′) for thepurpose of clarifying the influent water which containsnon-biodegradable and toxic matter and nutrients (N, P) and isintroduced into the anaerobic reactor (10) as described in Step 1, in aneven more efficient fashion. Next, the mixing means (14) mounted at themain shaft in the reactor is run continuously at the rat e of 3˜20 rpmfor generation of full fermentation. Here, in order to more efficientlyclarify the organic wastewater containing non-biodegradable and toxicmatter, nutrients such as nitrogen or phosphor, and heavy metals, asecondary fermentation process may further be included. To elaborate,the reaction water is firstly fermented selectively in said anaerobicreactor (10), and is then aerated in the aeration tank (20). Meanwhile,the return sludge containing some part of such sludge as discharged fromthe aerobic reactor (20) after the above aeration process and recycledthrough the internal return pump (60) is introduced into the otheranaerobic reactor (10′) through the inlet (11′″) and the introducingmeans (12′″). Then, the secondary fermentation is carried out bycontinuously running the mixing means (14′) with a rate of 3˜20 rpm forpreventing the channeling of the sludge which is apt to flow upwards.Such rate of mixing can prevent the channeling phenomenon of theinfluent admixture, which is caused due to the high concentration of thesludge when the admixture of the sludge and the organic wastewateruprises in the anaerobic reactor (10, 10′) during said reaction process.If the mixing rate of said mixing means (14, 14′) is less than 3 rpm,the reaction is rendered not sufficient due to the occurrence of thechanneling phenomenon. It is also undesirable that the mixing rate ismore than 20 rpm, for the complete mixing occurs in such a case, withthe result of incapability of taking advantage of the plug-flow typereactor.

Step 3

Influence into the Aerobic Reactor

Some portion of the sludge which is settled during the above process iscollected by the sludge-accumulating means (15) positioned at the lowercenter in the anaerobic reactor (10), and then discharged by theactuation force of the sludge-discharging pump (40). The reaction watercontaining some portion of the sludge which is flowing upwards isdischarge from the anaerobic reactor (10, 10′) and introduce into theaerobic reactor (20) via the discharging means (13, 13′) situated at theupper region in the anaerobic reactor (10, 10′).

Step 4

Aeration in the Aerobic Reactor

The reaction water containing some portion of the sludge, which wasdischarged from the anaerobic reactor (10, 10′) in the prior process, isprovided with a sufficient amount of oxygen through a diffuser (21). Assuch, the activities of the aerobic bacteria or the facultative bacteriapresent in the sludge are increased so that the various organic mattersin the wastewater are decomposed through oxidation to inorganic matterssuch as H₂O, CO₂, etch, the ammonia nitrogen and organic nitrogen aredegraded into the nitrate nitrogen, and the sludge comes to ingestexcess phosphor.

Step 5

Separation of Purified Water and Return of Sludge

The reaction water partially purified by the above aeration process istransferred into the terminal settling tank (30) for obtaining purifiedwater separately. The sludge settled by the gravity force is collectedby the sludge-accumulating means (15′) situated at the lower center inthe terminal settling tank (30). Afterwards, the collected sludge isreturned to the inlet (11, 11″) positioned at the external lowest regionof the anaerobic reactor (10) by the actuation force of thesludge-returning pump (50), and finally discharged to the outside fromthe anaerobic reactor (10) by the actuation force of thesludge-discharging pump (40) by way of a weight-reducing means such assludge digester or dewatering means.

In the above wastewater treatment method according to the presentinvention, suitable microorganisms for use in the anaerobic reactor (10,10′) or the aerobic reactor (20) may include Nitrosomonas, Nitrobacter,Denitrifier, Sulfate-reducing bacteria, Pseudomonas, Achromobacter,Aerhorbacter, Micrococcus, Bacillus, Proteus, Flovobacterium,Acinetobacter, Corynebacterium, Mycobacterium, or the like. In additionto these microorganisms, a variety of facultative bacteria, which arecommercially available, can be used in the present invention dependingon the target organic matter.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1a is a schematic diagram illustrating another embodiment of thewastewater treatment plant which employs the upflow anaerobic reactoraccording to the present invention.

FIG. 1b is a schematic diagram illustrating another embodiment of thewastewater treatment plant which employs the upflow anaerobic reactoraccording to the present invention.

FIG. 1c is a schematic diagram illustrating a third embodiment of thewastewater treatment plant which employs the upflow anaerobic reactoraccording to the present invention.

FIG. 2a is a schematic plane view of an introducing means in theanaerobic reactor according to the present invention.

FIG. 2b is a cross-sectional view of the introducing means shown in FIG.2a along the line A—A.

FIG. 2c is a cross-sectional view of the introducing means shown in FIG.2a along the line B-B.

FIG. 3a is a schematic plane view of a discharging means in theanaerobic reactor according to the present invention.

FIG. 3b is a cross-sectional view of the discharging means shown in FIG.3a along the line A—A.

FIG. 3c is a cross-sectional view of the discharging means shown in FIG.3a along the line B—B.

FIG. 4 is a graph showing the changes of COD concentration in theinfluent water and of the sludge concentration (suspended solids andvolatile suspended solids) in the anaerobic reactor, plotted with theincrease of organic matters in the water source.

FIG. 5 is a graph showing the changes of COD concentration in theinfluent water and of the sludge concentration (suspended solids andvolatile suspended solids) in the aerobic reactor, plotted with theincrease of organic matters in the water source.

FIG. 6 is a graph showing the changes of COD concentrations in theinfluent water and in each of the reaction water discharged from theanaerobic reactor and the aerobic reactor, respectively, plotted withthe increase of organic matters in the water source.

FIG. 7 is a graph showing the changes of the COD removal efficiency inthe anaerobic reactor and in the aerobic reactor, respectively, plottedwith the increase of organic matters in the water source.

FIG. 8 is a graph showing the changes of the concentrations oftrichlorophenol in the influent water and in each of the effluent waterdischarged from the anaerobic reactor and the aerobic reactor,respectively.

FIG. 9 is a graph showing the concentrations of sulfate which isinfluent into and effluent from the anaerobic reactor, respectively.

DESCRIPTION OF ILLUSTRATED EMBODIMENTS

The present invention will be described in further detail on the basisof the examples of embodiment given below. Since the examples ofembodiment are nothing but for explanation of the advantageous effect ofthe present invention, it will become apparent to those of ordinaryknowledge in the art that the scope of the present invention is neverlimited to these examples of embodiment in accordance with the gist ofthe present invention.

Example 1

Clarification of Organic Matter of High Concentration

The retention time of the organic matter of high concentration wasgoverned to be 15 hours in the whole reactors (anaerobic reactor: 7hours, aerobic reactor: 8 hours), and the concentration of the organicmatter was made to change from 600 mg/L to 2000 mg/L. With theseconditions, the clarification process was carried out by using thewastewater treatment plant comprising the upflow anaerobic reactoraccording to the present invention. As a result, the concentration ofthe sludge (suspended solids and volatile suspended solids) in theanaerobic reactor had been kept within the range of from 25000 mg/L to55000 mg/L, which concentration made it possible that the anaerobicreactor could serve as a thickening tank (ref.: FIG. 4). Theconcentration of the sludge (suspended solids and volatile suspendedsolids) in the aerobic reactor had changed within the range of 10000mg/L˜25000 mg/L according as the COD concentration of the influent waterwas changed from 600 mg/L to 2000 mg/L. That is, the concentration ofthe sludge in the present invention was kept more than 6˜8 times as highas that in the typical activated sludge method, viz., 1500 mg/L˜3000mg/L (ref.: FIG. 5). To this end, the bulk size of the reactor could bereduced in treating the organic matter of high concentration accordingto the method of the present invention since the greater amount of thesludge could be retained over the process when compared with the typicalactivated sludge process. Further, if the concentration of the organicmatter is increased, the sludge bulking befalls in the typical activatedsludge process, to which the process failure is attributed, whereas, inthe present invention, even in the case where the COD concentration ofthe influent water was increased to 2000 mg/L, the COD concentration inthe anaerobic reactor was kept below 100 mg/L and the removal efficiencyexhibited 85˜9% in the anaerobic reactor, and further, the CODconcentration of the effluent water from the aerobic reactor was alsokept below 100 mg/L to exhibit more than 98% of removal efficiency inthe aerobic reactor (ref.: FIGS. 6 and 7). Consequently, with theempirical confirmation of the fact that the wastewater containingorganic matter of high concentration could stably be treated, it hasbeen found that the present invention could also be applied to treatmentof organic wastewater of high concentration.

Example 2

Clarification of Wastewater Containing Trichlorophenol

The commonly known chlorinated compound, viz., trichlorophenol(hereinafter referred to as ‘TCP’) was used in the present Example as atoxic and non-biodegradable matter. Generally, the chlorinated compoundis known difficult to degrade using a biological treatment due to itsinherent toxicity and non-degradability. With increasing such toxicmatter TCP to 60 mg/L starting with 10 mg/L, the wastewater wasclarified by the anaerobic•aerobic method employing the upflow anaerobicreactor inoculated with the facultative bacteria according to thepresent invention. The result thereof, as can be seen in FIG. 8, was notonly that the intermediate products, which were typical of the generalanaerobic treatment, were not generated during the process in which TCPwas removed while passing through the upflow anaerobic reactor, but alsowas no intermediate product produced in the effluent water from theaerobic reactor.

Example 3

Clarification of Wastewater Containing Sulfate

600˜2400 mg/L of sulfate was fed into the wastewater treatment plantcomprising the upflow anaerobic reactor inoculated with the facultativebacteria according to the present invention. Then, the clarificationprocess was carried out over the period of about 100 days with 20 timesof measurement of the concentration of the influent liquor and that ofthe effluent liquor which was from the anaerobic reactor. As aconsequence, as illustrated in FIG. 9, the sulfate removal efficiency inthe anaerobic reactor was found to be 30˜60%. This proves that it isfeasible that the heavy metals are removed in the state of sulfate aftercombination with sulfides.

INDUSTRIAL APPLICABILITY

As particularly described and proved in the above, the present inventionprovides a wastewater treatment plant comprising an upflow anaerobicreactor and a wastewater treatment method using thereof With the use ofthe wastewater treatment plant comprising the upflow anaerobic reactorof the present invention, an organic wastewater containingnon-biodegradable and toxic matter, nutrients such as nitrogen orphosphor, and heavy metals can be biologically treated. Further, thebulk size of the reactor can be reduced by keeping a high concentrationof the sludge in the reactor, with the result of achievement of theeconomic treatment of the wastewater. In particular, the presentinvention can afford to exclude the need of a thickening tank in thecase of a sewage treatment plant thanks to the high concentration of thesludge in the upflow anaerobic reactor, and further, the initialsettling tank can also serve as a reactor, thereby achieving theadditional benefits of reduced land usage and reduced costs forconstruction and operation.

We claim:
 1. A wastewater treatment plant comprising, an upflowanaerobic reactor, an aerobic reactor, and a terminal settling tank,wherein said upflow anaerobic reactor includes an inlet mounted at anexternal lowest region of the anaerobic reactor; an introducing meansdirectly connected to said inlet and located at a lower region in theanaerobic reactor, and having a perforated drain pipe therein; mixingmeans which is provided at a main shaft thereof at a regular interval inthe anaerobic reactor; a discharging means having radial-shaped weirsextending outwards from a center and is positioned at an upper region inthe anaerobic reactor for allowing reaction water containing sludge froma reaction generated by said mixing means to flow into the aerobicreactor; a sludge-accumulating means which is provided at the lowerregion in the anaerobic reactor for collecting the sludge which issettled by gravity during said reaction by the mixing means to thecenter of the anaerobic reactor.
 2. The wastewater treatment plantaccording to claim 1, wherein said inlet is separated into an influentwater-guiding inlet and a return sludge-guiding-inlet, and saidintroducing means is separated into an influent water-introducing-meansand a return sludge-introducing-means.
 3. The wastewater treatment plantaccording to claim 1, further comprising a second upflow anaerobicreactor having an inlet provided at the external lowest region of theupflow anaerobic reactor for allowing both the reaction water which isdischarged from said anaerobic reactor and contains some portion of thesludge, and the return sludge which is returned from the anaerobicreactor by an internal return pump after the aeration process therein,to be mixed together and then flow into the lower region in the secondupflow anaerobic reactor; an introducing means provided at a lowerregion in the second upflow anaerobic reactor and directly connected tosaid inlet, and having a perforated drain pipe therein; a mixing meanswhich is installed at a main shaft at a regular interval in the secondanaerobic reactor; a discharging means which has radial-shaped weirsextending outwards from a center and is provided at the upper region inthe second anaerobic reactor for discharging the reaction watercontaining some portion of the sludge into the aerobic reactor.
 4. Amethod for treating wastewater with a wastewater treatment plant havingan upflow anaerobic reactor according to claim 1, comprising the stepsof: (i) introducing the influent water into the anaerobic reactorthrough the introducing means directly connected to the inlet, afterguiding said influent water into the anaerobic reactor through the inletprovided at the external lowest region of the anaerobic reactor; (ii)carrying out the fermentation reaction by continuously running themixing means at the rate of 3˜20 rpm for preventing a channelingphenomenon of the sludge tending to flow upwards, after introducing theactivated sludge into the anaerobic reactor through the inlet and theintroducing means; (iii) discharging the sludge settled in saidanaerobic reactor through the sludge-accumulating means provided at thelowest center in the anaerobic reactor, while discharging the upflowreaction water containing some portion of the sludge into the aerobicreactor from the anaerobic reactor through the discharging meansprovided at the upper region in the anaerobic reactor; (iv) aerating thereaction water which is discharged into the aerobic reactor in the priorprocess and contains some portion of the sludge, with a sufficientamount of oxygen by using a diffuser; and (v) transferring the reactionwater to the terminal settling tank after the above aeration process,obtaining the purified water separately, and returning the sludgesettled by the gravity force to the inlet provided at the externallowest region of the anaerobic reactor by using the actuation force ofthe sludge-returning pump after collecting said settled sludge in thesludge-accumulating means provided at the lowest center in the terminalsettling tank.
 5. The wastewater treatment method according to claim 4,comprising the steps of: (i) introducing both the reaction water whichis transferred from the anaerobic reactor and contains some portion ofthe sludge, and the return sludge which contains some portion of suchsludge as is recycled through the internal return pump after theaeration process in said aerobic reactor, into the additional anaerobicreactor through the inlet and the introducing means; (ii) carrying outthe secondary fermentation reaction by continuously running the mixingmeans at the rate of 3˜20 rpm for preventing the channeling phenomenonof the sludge tending to flow upwards; and (iii) discharging theadmixture into the aerobic reactor by using the discharging meansprovided in the anaerobic reactor.